Not 100% sure here as there seem to be many different types of underlay but as a general rule it always seems to me that if there is writting on it that faces up. Also if it's still rolled up it will unroll the correct way up, you'd have to turn it over otherwise.

I'm not a carpet layer so will bow to lymmranger's better knowledge, I was just going by observations made on sites.
Lymmy , can't help thinking that it all sounds a right ballache having to turn it all over , looking at the photo it'll unroll upside down i.e. blue side down or is it usd that way in other situations?

This is a closer look ladylola i think your right, or if not Carpet right that have done our main room have done it wrong they have done it blue side down but saying that my new underlay as got a waxing on the otherside were the main room is just foam and blue other side i could not help but take our carpet up to have a look a few minutes ago.

Normally (Cloud nine) the film side has the writing on it and would be film up. However Cloudnine as currently supplied is pants and most retailers use an alternative

With all the other manufacturers there is no true (impervious) film so actually it doesnt really matter. The effectiveness of the film is questionable anyway

This (PU) type of underlay is a source of great debate anyhow, some fitters claim it is too bouncy to give correct support on stair pads, I prefer it personally, and it has a great memory - dints caused by furniture seem to bounce out quite quickly